Various thermal transfer methods have been heretofore known. Of these, there has been proposed a method in which a sublimable dye is used as a recording agent and is supported on a substrate sheet such as a paper or a plastic film to prepare a thermal transfer sheet, and using the thermal transfer sheet, various full color images are formed on a thermal transfer image receiving sheet which is capable of being deposited with a sublimable dye, for example, a thermal image receiving sheet having a dye receptor layer on a paper or a plastic film.
In such a case, a thermal head of a printer is used as a heating means, so that a great number of color dots of three or four colors are transferred onto the thermal transfer image receiving sheet under heating for a short period of time, thereby to reproduce a full color image of an original. Such images as obtained above are very sharp because the used colorant is a dye, and are also excellent in transparency. Therefore, the images are excellent in half tone reproducibility and gradation properties, and are substantially the saxne as those formed by the conventional offset printing and gravure printing. Further, when the above image forming method is used, there can be formed images having high quality which are comparable to full color photographic images.
As the substrate sheet of the thermal transfer image receiving sheet used in the above sublimation type thermal transfer method, a plastic sheet, a laminate sheet of a plastic sheet and a paper, a synthetic paper, etc. are employed. However, in order to widely utilize the sublimation type thermal transfer method also in common offices, it is required to use ordinary papers such as a coat paper (i.e., art paper), a cast coat paper and a PPC paper as the substrate sheet of the image receiving sheet. In the case where such ordinary office papers are used as the substrate sheet and a dye receptor layer is formed thereon, there resides such a problem that when the paper is coated with an aqueous solution of a water-soluble resin or an aqueous emulsion of a water-insoluble resin to fill up the paper surface, water content is absorbed by a coat layer or a cast coat layer of the paper, resulting in waviness of the paper substrate in the drying procedure after the coating procedure. If the paper is coated with a solution of a hydrophobic resin, such problem hardly occur, but in this case other problems reside. That is, when a large amount of the solution is used in order to enhance the printed image quality, marked curling is brought about with moisture variation, because the pulp paper substrate has moisture absorption characteristics and the receptor layer is hydrophobic, etc., resulting in deterioration of printed image quality. Moreover, rubbing with a conveying roller during the printing procedure causes occurrence of paper powder.
Further, when the above-mentioned thermal transfer method is carried out, especially when an image having high gradation characteristics and shades of large difference is demanded, a large heat energy is out put from the thermal head within an area of high density, and thereby various problems occur. For example, the surface of the receptor layer suffers depressed and protruded portions, the substrate sheet of the thermal transfer image receiving sheet suffers thermal deformation in the excessive case, and curling is brought about on the thermal transfer image receiving sheet, whereby quality of the obtained image deteriorates. In the case of forming a full color image, printing procedures of 3 to 4 times are conducted on the same region of the receptor layer. Therefore, if the surface of the receptor layer is depressed and protruded, the transference of the dye in the second or the subsequent transferring stages is made ununiformly. As a result, the formation of an excellent full color image is hardly made, and deformation or curling of the thermal transfer image receiving sheet is much more strikingly brought about.
In addition, in the case of using the conventional thermal transfer image receiving sheets, there are such problems that the obtained printed materials are difficultly folded when they are intended to be folded or filed; they cannot be thinly folded even if the folding is possible; or they become bulky when filed, so that they are hardly applied to the ordinary office uses. Moreover, because of high cost and lacking of ordinary paper-like texture, they are unsuitable for ordinary office supplies.
In other conventional image receiving sheets in which the above-mentioned various substrate sheets are used and a dye receptor layer made of a thermal plastic resin such as a polyester resin, a vinyl chloride resin and a vinyl chloride/vinyl acetate copolymer resin is provided thereon, the dye receptor layer is easily peeled off due to the heat of the thermal head during the thermal transferring procedure or due to the adhesive tape.
For the formation of a sharp image, a sufficient whiteness of the dye receptor layer is necessary. However, when a large amount of a white pigment is introduced into the dye receptor layer for that purpose, deposition properties of the dye are decreased. Further, for obtaining an image of high resolution free from color dropout, decoloring, etc., the image receiving sheet is required to have sufficient cushioning properties so as to bring the dye receptor layer into good contact with the thermal head.
Such cushioning properties are generally obtained by forming an intermediate layer made of a resin having high cushioning properties between the substrate sheet and the receptor layer.
A most effective layer as the intermediate layer is a layer containing bubbles. In this case, however, when an image is formed by the thermal head, the bubbles contained in the intermediate layer are expanded again owing to the heat of the thermal head to make the surface of the receptor layer depressed and protruded or to break through the receptor layer, whereby the receptor layer becomes defective to give an adverse effect to the resulting image.
By providing the intermediate layer, the cushioning properties of the receptor layer can be improved, but the physical strength thereof is lowered. For example, if writing with a pencil or the like is intended to be made on the receptor layer before or after the image formation, a lead of the pencil scratches and writing is difficult because of low strength of the receptor layer. Otherwise, if the writing is compulsively made, the receptor layer is peeled off. In the case of using the ordinary paper such as a PPC paper as the substrate sheet of the image receiving sheet as described before, there is brought about such a problem that unevenness occur on the surface of the dye receptor layer correspondingly to the roughness of the surface of the paper substrate. For solving this problem, a transfer method in which the dye receptor layer is transferred onto the surface of the paper is known. In this method, a receptor layer-transfer film having a dye receptor layer and an adhesive layer laminated on a surface of a substrate film having high releasability is employed.
However, since the adhesive layer of the conventional receptor layer transfer films uses a heat-sensitive thermoplastic resin, the transference of the receptor layer needs application of heat, so that it is difficult to conduct high-speed transference. Further, in the case of using a coarse substrate sheet (e.g., paper) as the substrate sheet, adhesion strength thereof is insufficient in the high-speed transference. Moreover, the resulting image receiving sheet does not have satisfactory cushioning properties.
Among the thermal transfer image receiving sheets used in the above-mentioned thermal transfer methods, those having a dye receptor layer made of a thermoplastic resin on the surface of the substrate sheet require that an image of a dye is provided on the dye receptor layer. Therefore, a sensor for discriminating between a front surface and a back surface of the image receiving sheet is fitted to the thermal transfer device, and any one of the front and back surfaces of the image receiving sheet is provided with a detection mark capable of being detected by the sensor.
The detection of the front and back surfaces is made by a conventional optical means, so that on the image receiving sheet is formed a black or black-like detection mark having a reflectance largely different from that of the image receiving sheet. Accordingly, such detection mark exists on the image-formed surface, and thereby an appearance of the obtained image becomes bad.
Of course, the detection mark may be provided on the back surface of the image receiving sheet, but in this case, the detection mark can be seen through from the front surface, resulting in bad appearance of the obtained image. Moreover, in the case of forming the dye receptor layer on each surface side of the image receiving sheet, the same problem as described above still remains.
Formation of various information such as a photograph of face in the above thermal transfer methods is carried out by deposition of the dye within the card substrate, so that thus formed various information shows high smoothness, alter-preventing proper-ties and forgery-preventing properties. However, since the protective layer can be removed with a solvent, an acid, a base, etc., alteration or forging of photographs and other information is not completely prevented.